Eyes of MoUuscs and Arthropods. 661 



crease in tlieir luimber and perfection would be of correspondiug ad- 

 vautage to tlie animai , tlien tlie presence of a g-reat nuiuber of these 

 hii?lily developcd orgaus wonld offer no difticulty. If we su})pose that 

 these structures could, by a progressive series of chang-es, be trans- 

 formed into rudimentary eyes, the function of one not being lost until 

 after that of the other had been acquired, then we could ac- 

 count for the i)resence of an unnecessary number of eyes in any one 

 animai. 



As we hope to show more clearly in the last chapter, the so-called 

 eyes of Pecten , Area , Onc/ndnim and Cintoti are highly developed 

 heliophags. or organs for the absorption of energy from the sunlight. 

 The more light concentrated upon the energy-receiving surface , the 

 more benefit the animai would derive from the light. An increase in 

 the number of these orgaus would also be a great benefit, providedthe 

 animals were nocturnal, or lived in dark places. It is, therefore . on 

 this supposition , not stränge that some animals should bave a great 

 many such organs in a high stage of development, for every additional 

 organ , or every step toward a more perfect structure , would be of a 

 correspondiug advantage to the animai; we can therefore offer some 

 reasonable explanation for the great number of such organs present. 

 This. however. would be impossible on the supposition that they were 

 mere eyes. for it would be very difficult to offer any explanation of the 

 use Arca could make of two highly developed eyes , to say nothing of 

 the 1200 such organs that it possesses! The same reasoning would apply 

 to Pecten, with from 60 — 100 highly developed eyes, and OncJddium 

 or Chiton with several thousand eyes each. But if it is impossible to 

 offer any plausible Suggestion as to the present function of these eyes, 

 how is it possible to account for their first appearance, and for the in- 

 numerable intermediate stages through which they must necessarily bave 

 passed before reacliing their present condition? These difficulties are 

 obviated by supposing that they were, or are, heliophags; we can tlien 

 account for their great number and high development according to the 

 theory of naturai selection. But we know from experimeut, that Arca 

 can see — in the ordinary sense — that is, even a very small object will 

 cause muscular contraction. A heliophag absorbs light energy, and 

 therefore the most perfect forms bave lenses or refractive bodies for 

 concentrating the light , or are constructed in the most advantageous 

 way for its reception; but these are just the conditions that an eye has 

 to fulfil, so that the most perfect heliophag could at the same 

 time be an eye. Heliophags, like those of Arca and Acicula, receive 



